This invention relates generally to thermoelectric materials and more particularly to superlattice structures used in thermoelectric materials.
Superlattice structures, in general, are known and typically comprise a composite made of alternating ultrathin layers of different materials. Typically, the superlattice has an energy band structure which is different than, but related to, the energy band structure of the component materials. By the appropriate choice of materials (and other factors discussed below), a superlattice having a desired energy band structure and other characteristics can be produced.
Superlattices have many uses, including, but not limited to, use in the field of thermoelectric cooling materials. Whall and Parker have suggested using a superlattice prepared by molecular beam epitaxy (MBE) to enhance the thermoelectric figure of merit of a thermoelectric material. They expressed particular interest in silicon and SiGe alloys, but also mentioned the possibility of using PbTe, InAs and the transition silicates (e.g. Cobalt Si2). Specifically, a strained-layer superlattice consisting of 20 or more layers of Si and SiGe composition was suggested.
The use of MBE to grow superlattice structures (e.g., GaAs and AlAs) is disclosed in U.S. Pat. No. 4,261,771 issued to Dingle, et al. As disclosed in Dingle et al., the general technique for fabricating a superlattice by MBE comprises the following standard steps: (1) obtaining a substrate (e.g., from a commercial source); (2) cleaning a major surface of the substrate using standard preparation procedures; (3) placing the substrate in an evacuable metal chamber; (4) reducing the chamber pressure; (5) loading shuttered effusion cells (ovens) with the requisite source materials for growth; (6) heating the substrate to about 600 degrees C., to cause desorption of contaminants from the growth surface and then adjusting the substrate temperature to that desired for growth; (7) with the shutters closed, heating the ovens until the source materials vaporize; and (8) opening selected shutters to effect growth until the desired layer thickness is attained.
According to Dingle et al., the fabrication process can be described as forming a new composition of matter of A (e.g., GaAs) and B (e.g., AlAs or Ge) by directing a periodically pulsed molecular beam at a substrate. During the first part of each period an A-beam is directed at the substrate for a time effective to grow material A having a thickness of n monolayers and during the second part of each period directing a B-beam at the substrate for a time effective to grow-material B having a thickness of m monolayers.
Therefore, the fabrication of a superlattice by MBE, or other known epitaxial growth techniques, is generally known. However, the choice of materials and the relative amounts of the materials which make up the superlattice are predominant factors in determining the characteristics of the superlattice. For use as a thermoelectric material, it is desirable to choose the materials, and their relative amounts, so that the thermoelectric figure of merit is maximized.
The thermoelectric figure of merit (ZT) is a measure of the effectiveness of a material and is related to material properties by the following equation:
ZT=S2"sgr"T/K,
where S, "sgr", K, and T are the Seebeck coefficient, electrical conductivity, thermal conductivity and temperature, respectively. The Seebeck coefficient (S) is a measure of how readily electrons (or holes) can change energy in a temperature gradient as they move across a thermoelement, and is related to the strength of interaction of charge carriers with the lattice and the available energy states. The highest useful Seebeck coefficients are found in semiconductor materials with low crystal symmetry. In theory, to maximize ZT, one would try to maximize S, "sgr" and T and minimize K. However, in practice, this is not so simple. For example, as a material is doped to increase its electrical conductivity ("sgr"), bandfilling tends to lower S and the electron component, Ke of K increases. In most materials, ZT is maximized at doping levels approaching 1019 cmxe2x88x923. Since increasing (or decreasing) one parameter may adversely increase (or decrease) another parameter, it is important to select carefully the component materials to provide a high ZT. Currently, the best thermoelectric cooling materials have a ZT of approximately 1, at 300 K.
The figure of merit ZT is related to the thermoelectric materials factor (b*) where:
b*=xcexcm*3/2/KL;
where xcexc is the carrier mobility, m* is the density of states effective mass and KL is the lattice thermal conductivity. The precise relationship between b* and ZT is complex. However, for simplicity, it may be approximated as follows. If it assumed that b**=b*T5/2 and that there is one-band conduction, then ZT increases monotonically as b** increases.
A superlattice provides the opportunity to enhance ZT for a number of reasons. For example, it is known that the Seebeck coefficient increases as the period of a quasi-two-dimensional superlattice decreases. The electrical conductivity may be enhanced by means of modulation doping, which has been shown to increase the carrier mobilities in Si/SiGe strained-layer superlattices. Furthermore, the lattice thermal conductivity of a small-period superlattice is expected to be substantially lower than the average of the component materials because of augmented Umklapp phononxe2x80x94phonon scattering process (in which the total phonon vector is not conserved, but rather changes by 2xcfx80 times the reciprocal lattice vector) due to phonon-interface scattering effects.
In accordance with the present invention, a superlattice structure includes m monolayers of a first barrier material alternating with n monolayers of a second quantum well material with a pair of monolayers defining a superlattice period and each of the materials having a relatively smooth interface therebetween. Each of the quantum well layers have a thickness which is less than the thickness of the barrier layer by an amount which at least partially confines and in some instances causes substantial confinement of conduction carriers to the quantum well layer. The alternating layers of the first barrier material and the second quantum well material provide a superlattice structure having a figure of merit which increases with increasing temperature beyond that achievable with bulk materials. With this particular arrangement, a superlattice structure which may be used for various purposes including, but without limitation, thermoelectric power generation, thermoelectric cooling, electronic devices and electrooptic devices. For example, a superlattice according to the present invention may be made of alternating layers of a first material and a second material, where the first material comprises PbTe and where the second material comprises materials selected from the group consisting of Pb, Sn, Ge, Te, Se, S, Sb, As or P. For example, but without limitation, the superlattice may be made of alternating monolayers of PbTe and PbEuTe. More particularly, the superlattice may be provided from alternating layers of (PbEuTe)m and (PbTe)n (where m and n are the number of PbEuTe and PbTe monolayers per superlattice period, respectively) having engineered electronic structures for improved thermoelectric power generation materials and other uses. Such superlattices may be made fabricated via epitaxial techniques such as by molecular beam epitaxial (MBE) growth. In one particular embodiment, the superlattice may be provided from m layers of PbEu0.8Te0.2 and n layers of PbTe, where m and n are preferably between 10 and 2000. However, the invention is not so limited. Other combinations and amounts of similar materials may be used. According to a preferred embodiment, MBE may be used to grow PbEuTe and PbTe on (111) barium fluoride (BaF2) substrates at substrate temperatures in the range of 100xc2x0 to 250xc2x0 C.
In accordance with a further aspect of the present invention, a superlattice structure for a thermoelectric element used for thermoelectric power generation, includes a plurality of epitaxially grown alternating layers of materials A and B, where materials A and B are substantially lattice matched in a direction perpendicular to the direction of growth and are formed from materials which provide a thermoelectric figure of merit greater than 1.7 and which increases with increasing temperature. With this particular arrangement, a superlattice structure having thermo-electric figure of merit which increases to a value greater than achievable with bulk materials is provided. In one particular embodiment, each layer formed from material A corresponds to a barrier layer comprising Pb1-y Euy Te wherein 0xe2x89xa6yxe2x89xa61 and each layer formed from material B corresponds to a quantum well layer comprising PbTe. The alternating layers of materials A and B are epitaxially grown over a buffer layer. In one embodiment, the buffer layer includes Pb1-y Euy Te wherein 0xe2x89xa6yxe2x89xa61. The superlattice and buffer layers may be grown, deposited or otherwise disposed over a substrate provided from BaF2. The structures disclosed herein indicate an enhanced ZT at 300 K in thermoelectric quantum wells, which is believed to be a result of the large density-of-states per unit volume increase that occurs for small well widths in a quasi-2D geometry. Measured Seebeck coefficients at a temperature of about 300 K of Pb0.927Eu0.073Te/PbTe MQW samples with near-optimum carrier concentrations are about two times larger than those of high-quality bulk PbTe for quantum-well widths in the 17 to 23 xc3x85 range. The 300 K electron mobility of a sample with a well width of 19 xc3x85 is 15 to 20% higher than lower-doped single-layer MBE-grown PbTe layers, which indicates the effectiveness of modulation doping utilized in the sample. The combined enhancements of the Seebeck coefficient and mobility resulted in an increase in the 300 K ZT of PbTe from xcx9c0.45 for bulk material to greater than 1.2 in quantum wells.